The operating principle of the invention is explained with respect to the heater of an oxygen probe. The application of the invention is however not limited to the application in connection with the oxygen probe but is always then applicable when heaters having a temperature-dependent resistance are to be checked with the aid of a temperature sensor mounted at a remote location.
The oxygen content of the exhaust gas is determined with the oxygen probe and the value determined in this manner is supplied to a control arrangement which operates to adjust a pregiven air/fuel ratio. The oxygen probe is only operationally ready above a minimum operating temperature. In this way, the control of the air/fuel mixture by means of the oxygen probe is only then possible when the oxygen probe has reached its operating temperature. Only then can a control to an optimal air/fuel mixture take place, for example, with respect to a low emission of toxic material. The operating temperature of the oxygen probe should be reached as rapidly as possible after the internal combustion engine is started in order to hold the emission values low. The heat-up of the oxygen probe takes place by means of the exhaust gases of the engine and is accelerated by the electric oxygen probe heater for the reasons delineated above. The electric oxygen probe heater is also then necessary when, for example, the heating capacity of the exhaust gas is inadequate such as during idle in order to maintain the oxygen probe at the operating temperature or for overrun operation which takes a longer time.
It is necessary to check the operability of the oxygen probe heater in the context of providing a low emission of toxic materials. Many methods are known in order to recognize one or several fault conditions such as interruptions, short circuits and shunts. The test of the operability of the oxygen probe heater takes place in a number of ways, for example: via the current flow through the oxygen probe heater detected by means of the measuring resistance as disclosed in United States patent application Ser. No. 07/862,567, filed Jun. 22, 1992, still pending; via the output signals of the oxygen probe as disclosed in U.S. Pat. Nos. 4,170,967 and 5,054,452; via the heat-up performance of the oxygen probe as disclosed in U.S. Pat. No. 5,090,387; or, via the oxygen probe temperature which can be determined in various ways such as from the internal resistance of the oxygen probe as disclosed in U.S. Pat. No. 4,419,190 or with a temperature sensor as disclosed in U.S. Pat. No. 3,915,828.
The methods recited above have the disadvantage that only a severe malfunction of the oxygen probe heater can be detected. The precise value of the electrical resistance of the oxygen probe heater is usually not included in the check of operability since the electrical resistance varies considerably in dependence upon temperature even for an oxygen probe heater which is fully operational and, for this reason, a relatively large desired value interval must be pregiven. In this way, only such deviations of the resistance from the desired value can be reliably detected which are greater than the resistance changes caused by temperature fluctuations and occurring during normal operation. A changed resistance can however lead to a lower heating capacity of the oxygen probe heater and therefore lead to a longer heat-up time of the oxygen probe. In this way, the toxic material emission of the engine increases since the time duration becomes longer until the oxygen probe can take over the control function for which it is provided. The reverse case, namely an increased heating capacity, can lead to damage of the oxygen probe and/or of the oxygen probe heater by exceeding the maximum permissible operating temperature.